Part of a natural cyclical change in the prevailing environmental conditions on the surface of a planet, over the course of a complete orbit round the Sun. A planet experiences seasons if its rotation axis is not at 90° to the plane of the ecliptic. Seasonal effects, particularly on the polar ice caps, are quite marked on the Earth and Mars. Conventionally, four seasons are identified - spring, summer, autumn and winter - but there are no strict divisions between them and seasonal conditions may vary considerably from year to year.